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Technical Paper

Combustion Chambers for Natural Gas SI Engines Part I: Fluid Flow and Combustion

The most economical way to convert truck and bus DI-diesel engines to natural gas operation is to replace the injector with a spark plug and modify the combustion chamber in the piston crown for spark ignition operation. The modification of the piston crown should give a geometry well suited for spark ignition operation with the original swirling inlet port. Ten different geometries were tried on a converted VOLVO TD102 engine and a remarkably large difference in the rate of combustion was noted between the chambers. To find an explanation for this difference a cycle resolved measurement of the in-cylinder mean velocity and turbulence was performed with Laser Doppler Velocimetry (LDV). The results show a high correlation between in cylinder turbulence and rate of heat release in the main part of combustion.
Technical Paper

Experiments and Simulation of a Six-Cylinder Homogeneous Charge Compression Ignition (HCCI) Engine

A 6-cylinder truck engine was modified to run in HCCI-mode. The aim was to show whether or not it is possible having HCCI run a multi-cylinder engine, to provide brake values of emissions and efficiency and to verify models for engine system simulation. The work proved that it is feasible to use HCCI in multi-cylinder engines with high brake efficiency. Emissions' strong dependence on inlet temperature and octane number was demonstrated. The numerical models simulated the mean effective pressure with high precision, while inlet and exhaust pressures were less accurate, mainly due to the limitations of the turbo maps used.
Technical Paper

The Effect of Displacement on Air-Diluted Multi-Cylinder HCCI Engine Performance

The main benefit of HCCI engines compared to SI engines is improved fuel economy. The drawback is the diluted combustion with a substantially smaller operating range if not some kind of supercharging is used. The reasons for the higher brake efficiency in HCCI engines can be summarized in lower pumping losses and higher thermodynamic efficiency, due to higher compression ratio and higher ratio of specific heats if air is used as dilution. In the low load operating range, where HCCI today is mainly used, other parameters as friction losses, and cooling losses have a large impact on the achieved brake efficiency. To initiate the auto ignition of the in-cylinder charge a certain temperature and pressure have to be reached for a specific fuel. In an engine with high in-cylinder cooling losses the initial charge temperature before compression has to be higher than on an engine with less heat transfer.
Technical Paper

Comparison Between In-Cylinder PIV Measurements, CFD Simulations and Steady-Flow Impulse Torque Swirl Meter Measurements

In-cylinder flow measurements, conventional swirl measurements and CFD-simulations have been performed and then compared. The engine studied is a single cylinder version of a Scania D12 that represents a modern heavy-duty truck size engine. Bowditch type optical access and flat piston is used. The cylinder head was also measured in a steady-flow impulse torque swirl meter. From the two-dimensional flow-field, which was measured in the interval from -200° ATDC to 65° ATDC at two different positions from the cylinder head, calculations of the vorticity, turbulence and swirl were made. A maximum in swirl occurs at about 50° before TDC while the maximum vorticity and turbulence occurs somewhat later during the compression stroke. The swirl centre is also seen moving around and it does not coincide with the geometrical centre of the cylinder. The simulated flow-field shows similar behaviour as that seen in the measurements.